With increasing automation of various processes, speed control of electrical induction motors has become of increasing interest. Asynchronous induction motors can vary in speed only upon variation of the frequency of the electronic power supplied to the motor. The electronic power is derived from the network at a fixed network frequency (usually 50 or 60 Hz) and is converted to a variable frequency alternating current in a so-called "inverter". The inverter generally converts the network alternating current into a direct current and, in turn, the direct current into an alternating current whose frequency can be controlled by controlling the conductivity timing of the power transistors of the inverter.
For an induction motor to be operated correctly, it is known that saturation of the magnetic core should be avoided under all supply conditions, but that the motor should be energized with the maximum magnetic flux short of saturation for optimum mechanical efficiency.
The voltage supplied to the motor cannot be maintained constant while the frequency is varied, but rather must increase approximately linearly with a linear increase in frequency and hence in speed. This relationship is due to the fact that electromotive counterforce (back EMF) generated by the motor which counteracts the applied supply voltage must, by reason of LENZ's Law, increase with the increased frequency. The magnetic flux in the magnetic circuit should be maintained constant. The magnetic flux is proportional to the applied voltage and inversely proportional to the frequency thereof. By varying the frequency, the speed of the rotating field can be regulated and the mechanical speed of the motor determined thereby.
An inverter capable of delivering the alternating supply voltage with variable frequency to an induction motor, comprises generally the aforementioned rectifier stage which can include a condenser or capacitor functioning as a filter for converting the alternating voltage of the supply network to direct current, power transistors which can be of the MOSFET (Metal oxide Semiconductor Field Effect Transistor) or IGBT (Insulated Gate Bipolar Transistor) type in pairs in respective arms connected across the direct current terminals of the rectifier circuit and having outputs tapped between the power transistors of each arm to supply the electric motor.
Diodes can be provided for energy recovery and serve to protect the field effect transistors against reverse current flows and transients, i.e. transient voltage surges. A control circuit can be connected to the gates of the MOSFETs or IGBTs to trigger them in the corresponding sequence to energize the motor with alternating current, the frequency of such energization determining the motor speed. A piloting unit can be provided for responding to the control unit to produce the gate triggering signals for the power transistors and a power supply can be provided for these units, connected across the rectifier output terminals. With the appropriate sequence signals generated by the control unit and conditioned by the piloting unit, the MOSFETs or IGBTs can be rendered conductive in the appropriate sequence to provide a polyphase or monophase energization of the motor.
In the case of a three-phase system, utilizing, for example, a three-phase induction motor whose windings are connected in a delta (.DELTA.) connection, the signals can have square wave configuration with voltages passing between +Vat and -Vat with a phase displacement of 120.degree.. The three-phase inverter can have three arms of the type described and are triggered in 120.degree. out-of-phase relationship but operated at the same cadence or frequency determining the motor speed.
It has been proposed, in this regard, to vary the voltage as a function of the frequency by chopping the piloting signals, thereby resorting to a pulse width modulation of the trigger signals for the power transistors. In pulse width modulation (PWM) the widths of pulses applied to the gates can be varied.
In general, therefore, it can be said that it is possible to obtain various voltages at the outputs of the variable inverter connected to the motor by varying the number, width and intervals of the piloting pulses. The frequency of triggering of the power transistors for motor control purposes is usually between 1 and 16 Hz. Because of the time constant of the motor windings and the operation in a frequency range between 1 and 16 Hz, energization of the motor is within a range ready perceptible to the human ear. This fact and the fact that the magnetostriction phenomena in the magnetic domains of the motor causes the typical hissing which characterizes induction motors whose speeds are controlled by frequency variation.